[LTP] [PATCH v3 4/4] syscalls/msgstress: tune limit of processes for small machines

[Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>]

Forking the exactly amount of processes as the limit (either from
max_pids or from cgroups) is risky - OS might be doing some work and
interfere with the test.  Instead leave some reserve (hard-coded to
500) for the OS so the test won't fail on fork failure.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
---
 lib/tst_pid.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/tst_pid.c b/lib/tst_pid.c
index c1ea3fe90e83..1b2e85959430 100644
--- a/lib/tst_pid.c
+++ b/lib/tst_pid.c
@@ -32,6 +32,8 @@
 #define PID_MAX_PATH "/proc/sys/kernel/pid_max"
 #define CGROUPS_V1_SLICE_FMT "/sys/fs/cgroup/pids/user.slice/user-%d.slice/pids.max"
 #define CGROUPS_V2_SLICE_FMT "/sys/fs/cgroup/user.slice/user-%d.slice/pids.max"
+/* Leave some available processes for the OS */
+#define PIDS_RESERVE 500
 
 pid_t tst_get_unused_pid_(void (*cleanup_fn) (void))
 {
@@ -106,7 +108,7 @@ static int get_session_pids_limit(void (*cleanup_fn) (void))
 	if (max_pids < 0)
 		return -1;
 
-	return max_pids;
+	return max_pids > PIDS_RESERVE ? max_pids - PIDS_RESERVE : 0;
 }
 
 int tst_get_free_pids_(void (*cleanup_fn) (void))
-- 
2.27.0